Modeling the Dielectric Properties of Minerals from Crystals to Bulk Powders for Improved Interpretation of Asteroid Radar Observations

1,2D. C. Hickson,3A. L. Boivin,4C. A. Tsai,1M. G. Daly,3R. R. Ghent
Journal of Geophysical Research (Planets) (In Press) Link to Article [https://doi.org/10.1029/2019JE006141]
1Centre for Research in Earth and Space Science, York University, Toronto, ON, Canada
2Arecibo Observatory, University of Central Florida, PR, USA
3Solar System Exploration Group, Department of Earth Sciences, University of Toronto, Toronto, ON, Canada
4Department of Physics, University of Toronto, Toronto, ON, Canada
Published by arrangement with John Wiley & Sons

Planetary radar has provided a growing number of datasets on the inner planets and near‐Earth and main‐belt asteroid populations in the solar system. Physical interpretation of radar data for inference of surface properties requires constraints on the constitutive parameters of the material making up a given surface. In this study, the complex permittivity of seven minerals as a function of frequency and porosity is measured using the coaxial transmission line method to determine the mixing equation that best describes the relationship between the real part of the complex permittivity of single mineral crystals and granular mineral powders. We find the Looyenga‐Landau‐Lifshitz and Bruggeman Symmetric mixing equations to describe our experimental results with the highest accuracy. The variation in the real part of the permittivity of solid mineral crystals between different minerals is shown to depend on the grain density and the chemical composition of the minerals. These mixing relationships are incorporated into an asteroid radar model and used to calculate the porosity in the near‐surface of seven asteroids visited by robotic spacecraft using Earth‐based radar observations. The results of the asteroid radar model support the presence of significant porosity in the boulders on the surface of asteroid 101955 Bennu. This research highlights the ability of radar to measure the porosity on asteroid surfaces and provides theoretical and experimental justification for the inversion of permittivity to bulk density assumed by the asteroid radar model.

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